A microbial fuel cell is equipped with a pair of electrodes, an external circuit for electrically connecting the electrodes, and a separating film for separating the pair of the electrodes. Microorganisms with an extracellular electron transfer capability, such as Shewanella or the like are held on one electrode. Here, the extracellular electron transfer capability means the capability to obtain biogenic electric energy and at the same time transfer electrons to electron accepters by utilizing metallic ions and their oxidative products as the electron accepters to reduce the ions and the products (see nonpatent document 1). The forgoing extracellular electron transfer capability is found in part of bacteria acting as microorganisms. That is, a specific electron transfer mechanism that discharges electrons to the outsides of microorganisms via cytochrome localized in a cell membrane is found in part of bacteria, such as a Shewanella group (hereunder, simply referred to as Shewanella) such as Shewanella loihica and Shewanella oneidensis, a Geobacter group, a Rhodoferax group, a Pseudomonas group or the like (see nonpatent documents 2, 3, 4).
The microbial fuel cell thus structured is a device in which microorganisms transfer electrons to the above one electrode, thereby producing electric energy. As fuel for producing electric energy, a regenerable biomass and organic pollutants contained in human sewage or the like can be utilized and hence the microbial fuel cell has recently received widespread attention as a sustainable energy source. Besides, there also exist microorganisms with a capability to reduce and fixate a metallic element, which has been noted as means for sewage treatment and environmental purification, too. The microbial fuel cell falls into the two general classifications which are a system for employing a single kind of microorganism, and a mixed bacterial culture system for directly employing a microbial group living in sewage or the like (e.g., see patent document 1).
Shewanella is a microorganism most extensively employed in the former system.
Patent document 1: Japanese unexamined patent application publication No. 2006-81963.
Nonpatent document 1=Lovely D. R.; Nat. Rev. Microbiol., 2006, 4, 497-508
Nonpatent document 2: Gralnick, J. A.; Newman, D. K.; Molecul. Microbiol. 2007, 65, 1-11
Nonpatent document 3: Hernandez, M. E.; Newman, D. K.; Cell. Mol. Life Sci. 2001. 58, 1562-1571
Nonpatent document 4: Journal of Japanese Society of Microbial Ecology (2008) Vol. 23, No. 2, P. 58